[PATCH 5/6] rcu: Remove full memory barrier on RCU stall printout

Frederic Weisbecker
2024-05-15T12:53:49Z

RCU stall printout fetches the EQS state of a CPU with a preceding full
memory barrier. However there is nothing to order this read against at
this debugging stage. It is inherently racy when performed remotely.

Do a plain read instead.

This was the last user of rcu_dynticks_snap().

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree.c | 10 ----------
kernel/rcu/tree_stall.h | 4 ++--
2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 02f6f3483482..04dde7473613 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -299,16 +299,6 @@ static void rcu_dynticks_eqs_online(void)
ct_state_inc(RCU_DYNTICKS_IDX);
}

-/*
- * Snapshot the ->dynticks counter with full ordering so as to allow
- * stable comparison of this counter with past and future snapshots.
- */
-static int rcu_dynticks_snap(int cpu)
-{
- smp_mb(); // Fundamental RCU ordering guarantee.
- return ct_dynticks_cpu_acquire(cpu);
-}
-
/*
* Return true if the snapshot returned from rcu_dynticks_snap()
* indicates that RCU is in an extended quiescent state.
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 460efecd077b..4b0e9d7c4c68 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -501,7 +501,7 @@ static void print_cpu_stall_info(int cpu)
}
delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
falsepositive = rcu_is_gp_kthread_starving(NULL) &&
- rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
+ rcu_dynticks_in_eqs(ct_dynticks_cpu(cpu));
rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
if (rcuc_starved)
// Print signed value, as negative values indicate a probable bug.
@@ -515,7 +515,7 @@ static void print_cpu_stall_info(int cpu)
rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
"!."[!delta],
ticks_value, ticks_title,
- rcu_dynticks_snap(cpu) & 0xffff,
+ ct_dynticks_cpu(cpu) & 0xffff,
ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),
rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
--
2.44.0